The coatings industry has long sought to develop coating compositions that can be applied to wood surfaces to provide air drying coatings which demonstrate long-term water repellency and durability on exterior exposure.
It is known that the presence of a wax in a coating provides water repellency. In certain prior coatings, a paraffin wax was melted, dissolved in an organic solvent and incorporated into the organic solvent-based coating composition as a solution. However, only a small proportion of wax can be incorporated in this manner. This is unsuitable for many exterior applications where larger amounts of wax are needed to extend the duration of water repellency.
Organic solvent-based coating compositions have been developed which include a large proportion of paraffin wax and which provide coatings with the foregoing properties of water repellency and durability (see U.S. Pat. No. 4,450,247 which issued on May 22, 1984 and which is assigned to DeSoto, Inc., the assignee of this invention). Such coating compositions include a solution of an alkyd resin in mineral spirits.
But the use of large amounts of organic solvents in such coating compositions leads to numerous environmental and other known disadvantages because of the presence of a solvent. It is desirable to minimize these disadvantages through the use of coatings which contain minimal amounts of volatile organic solvents. Thus, a need exists for aqueous coatings which substantially duplicate the water repellency and the durability of the patented solvent-based coatings.
In U.S. application Ser. No. 681,207, filed Dec. 13, 1984 (now U.S. Pat. No. 4,576,987 issued Mar. 18, 1986) which is also assigned to DeSoto, Inc., we disclose aqueous, air drying water-repellent latex coating compositions that include an aqueous microcrystalline wax dispersion, an aqueous latex of emulsion copolymerized monoethylenically unsaturated monomers and an aqueous dispersion of an N-methylol functional self-crosslinking polyurethane.
The polyurethane component provides several functions, one of which is to toughen the coating. This is helpful when the copolymerized unsaturated monomers are of the type that do not readily dry in air to form hard films in the absence of a curing agent. For example, certain polymers of monoethylenically unsaturated monomers in aqueous emulsion do not coalesce adequately at low temperatures (less than about 20 degrees C.) so when these polymers are softened for adequate coalescence, they require the presence of a curing agent to provide the desired film hardness. The use of such a curing agent, however, adds to the cost of the coating composition, and it impairs the adhesion needed when largely unpigmented coatings are used on decks.
It is desirable, therefore, to provide aqueous latex coating compositions which dry under ambient conditions to form durable water-repellent films and which include a large proportion of wax, but which do not include substantial amounts of organic solvents or curing agents. These coating compositions should coalesce below about 25 degrees C. and preferably below about 20 degrees C.
Moreover, and even when the copolymer has sufficient hardness, it is desired to provide improved adhesion and wear resistance for deck application.